This is a research study which aims to improve the way that doctors determine the dose of chemotherapy given to patients. Right now, chemotherapy dose is determined by a patient+s height and weight. However, some patients metabolize chemotherapy faster or slower than the average person because of a different level of drug metabolizing enzyme in the liver. Therefore, some patients are either given too small or too large a dose of chemotherapy because the amount of enzyme is not taken into account. This research study will examine the use of a simple test, called the erythromycin breath test, to determine the amount of enzyme which can metabolize the chemotherapy drug, docetaxel (Taxotere). Docetaxel is in general use all over the country for Stage IV breast cancer, and is given in a standard dose to patients. However, in this study, the dose of docetaxel will be tailored to the amount of enzyme which is available in the participant to metabolize the drug. The level of docetaxel in the blood will be tested after the first dose is given. The goal of the study is to reduce the wide variation in blood levels which is often seen after docetaxel is given. In this study, docetaxel will be given along with another drug, trastuzumab (Herceptin), which is a manufactured antibody against a breast cancer growth factor receptor known as Her-2 neu. All of the participants in this trial will have high expression of Her-2 neu on their tumors. Another goal of the study is to find out how many patients have cancer shrinkage with tailored-dose docetaxel plus trastuzumab.Approximately 30 women with breast cancer will be recruited at the University of Michigan. The women eligible for this study must not have received docetaxel in the past. Research subjects must be in fairly good health, as determined by blood tests and physical exam. The cancer must be able to be measured on x-rays or scans.